Wireless communications access, on which our society and economy is growing increasingly dependent, is becoming pervasive in all aspects of daily societal functions. For example, wireless communication has become increasingly available to users on board mobile platforms such as land vehicles, aircraft, spacecraft, watercraft or the like. Wireless communication services for passengers of mobile platforms include Internet access, e.g., e-mail and web browsing, live television, voice services, virtual private network access and other interactive and real time services.
Wireless communication platforms for remote, hard to access, or mobile user terminals, e.g., mobile platforms, often use communication satellites that can provide service coverage over large geographic footprints, often including remote land-based or water-based regions. Generally, base stations (e.g., ground base stations) send information (e.g., data) to the user terminals through a bent pipe via one or more satellites. More specifically, the base stations send information on a forward link to the satellite that receives, amplifies and re-transmits the information to an antenna of one or more fixed or mobile user terminals. The user terminals, in turn, can send data back to the base stations via the satellite. The base stations can provide the user terminals with links to the Internet, public switched telephone networks, and/or other public or private networks, servers and services.
Modern satellites and other cellular communication systems often employ a number of spot beams providing a beam laydown that forms coverage over a geographic region that may be divided into a plurality of cells. In a communication system using spot beams, the same frequency may be used at the same time in two or more cells. These beams may be configured to maintain a predetermined co-polar isolation (e.g., carrier-to-interference ratio) value in order to minimize the interference among beams. This is called spatial isolation and spatial reuse. In one typical parlance, each spot beam may be assigned a color to create a color pattern that matches a frequency reuse pattern. Identical frequencies, then, may be reused by different beams with the same color.
A number of systems use beamforming antennas to suppress interference by employing beam designs with low sidelobes or adaptive beamforming techniques. For these systems, the beamformer may be implemented onboard the satellite (sometimes referred to as an onboard beamformer—OBBF) or on the ground (sometimes referred to as a ground-based beamformer—GBBF). There are important differences between systems that employ one of these techniques. An OBBF may be constrained by size and power dissipation requirements of the satellite, which may make it difficult to implement sophisticated beamforming algorithms. A GBBF on the other hand may not have similar size and power constraints, and may therefore be capable of implementing sophisticated beamforming algorithms, such as those that include adaptive interference cancellation schemes. But GBBF-based systems require much higher feeder bandwidth, which may render them infeasible in some situations.
The performance requirements of a system often assume that the satellite's deployed reflector antenna has an ideal surface (ideally-shaped). But the larger surfaces of some reflectors and their deployment often create distortion that results in a non-ideal surface (non-ideally-shaped). And when the surface deviates from its ideal shape, performance may degrade. Likewise, thermal effects may degrade performance. Adaptive beamforming techniques may reduce the impact of a non-ideally-shaped reflector and thermal effects. But this is currently limited to GBBF-based systems and may at least partially explain why these systems are often more desirable than OBBF-based systems for the purpose of interference suppression.